1. Field
The field relates to an image processing apparatus and an image processing method.
2. Description of the Related Art
A digital camera converts an image photographed by an image pickup device to a digital image. In digital cameras in recent years, the photodetection capacity of its image pickup device has been improved so that random noise as well as light from a photographic object is easier to detect thereby reducing S/N ratio. For the reason, the digital cameras in recent years have been demanded to improve the S/N ratio by reducing the random noise and the like while keeping the quality of a photographed image from dropping.
Generally, as an image processing apparatus for improving the S/N ratio by reducing the random noise and the like, apparatuses using a spatial filter provided with a weight table or a median filter have been known. As disclosed in for example, “Image Processing Applied Technology” (written by HIROSHI TANAKA, issued by Kogyo Chosakai Publishing, Inc., June 1991, 3rd edition), the aforementioned spatial filter carries out product sum operation of 3×3 region pixel value within digital image and 3×3 region weight table in succession over entire area of digital image so as to calculate an average of each product sum operation successively. Then, in the aforementioned spatial filter, with each calculated average employed as a pixel value of a target pixel in 3×3 region, the digital image is corrected according to each average calculated over the entire area of the digital image. Then, because the digital image is corrected according to each average in the aforementioned spatial filter, random noise mixed in digital image before correction is smoothed. Thus, random noise mixed in the digital image before correction is reduced in the aforementioned spatial filter so as to improve the S/N ratio.
As disclosed in, for example, “Easily Understandable Digital Image Technology” (written by HITOSHI KIYA, first edition, issued by CQ Publishing Co., Ltd., February 1996), in the aforementioned median filter, all pixel values in the 3×3 region within the digital image are rearranged in descending order so as to obtain a median value (intermediate value) within the rearranged value as a pixel value of the target pixel in the 3×3 region. Then, in this median filter, the median value is obtained over the entire area of the digital image and the digital image is corrected with each median value. Then, because the digital image is corrected by each median value in this median filter, noise mixed in the digital image is suppressed as compared with a case where the digital image is formed with a maximum value of the rearranged values. Thus, this median filter can improve the S/N ratio by correcting the digital image with the median value.
Further, as an image processing apparatus for improving the S/N ratio, a type using ε filter has been known (see “ε-separation nonlinear digital filter and its application”, by HIROSHI HARASHIMA and other three persons, 1982, SHINGAKURON, vol. J65-A, no. 4). The ε filter has a characteristic of removing small amplitude random noise added to a signal while maintaining an accidental large-amplitude change component in the signal. Then, the ε filter can remove the random noise added to an optical signal from an object so as to improve the S/N ratio.